Electrical loads for vehicles continue to escalate. At the same time, the overall package size available for the electrical generator continues to shrink. Consequently there is a need for a higher power density system and method of generating on-board electricity.
The increase in the demand for current to be produced by an alternator leads to a need for corresponding adaptation of the system for current regulation, and particularly of a system whereby the three-phase alternating current produced by an alternator is rectified, into a direct current, which can be stored in a battery of a vehicle or be used directly by the electrical circuit of the vehicle which is supplied with a direct current (DC) voltage.
Rectification of the three-phase alternating current is generally obtained by means of a rectifier bridge having between six and twelve power diodes depending on the application. Three of these diodes in a six diode configuration are the positive diodes, and are connected between the phase terminals of the stator windings of the alternator and the positive terminal B+ of the alternator which is connected to the battery and the electrical circuit of the vehicle. Three further diodes, namely the negative diodes, are connected between electrical ground or earth of the vehicle and the aforementioned phase terminals of the stator windings.
The diodes constitute the rectifier bridge and are subjected to high current. Hence, it is necessary to cool them in the most effective way possible. To this end, it is know to install the diodes on metal carrier plates, which are located on the outside of the alternator and which constitute a dissipator for the heat produced by the diodes. The diodes are grouped on two carrier plates, one of which is reserved for the positive diodes, and the other for the negative diodes.
The rectifier diodes are collected to respective metal carrier plates, and these carrier plates are used as heat sinks for these diodes as well. The rectifier diodes are inserted by pressure in receiving bore holes of the metal carrier plate or heat sink, or are soldered to the metal carrier plate using appropriate solder alloys. The end wires connected to the rectifier diodes enable the rectifier diodes to be connected to external sources.
However, under certain particularly severe operating conditions, it has become apparent that this cooling of the diodes, and in general terms the cooling of the whole of the current regulating means, can be insufficient to ensure long-term reliability of the alternator.
The heat sinks are typically constricted in the shape of a circle or crescent and are fastened in the same plane to the alternating current generator.
It is important that the bridge rectifiers must not only be able to withstand normal battery charging current, but must also be able to supply current, perhaps as much as ten times the normal charging current. Bridge rectifiers, as discussed, are typically unable to absorb or conduct these types of excess currents and are also unable to rapidly dissipate the resulting heat. In order for bridge rectifiers to handle these types of excessive currents and heat, it becomes necessary to utilize a bridge rectifier which has a higher current handling capability. Due to the space limitations of the alternating current generator, it then becomes very difficult to provide such a bridge rectifier from a feasibility standpoint as well as at an economical cost.
Increasing the current capacity and heat dissipating characteristics of the bridge rectifier has included mounting of semiconductor diode chips onto first and second metallic heat sinks which are electrically insulated from each other by a thin sheet of electrical insulating material. The diode chips are then covered by a protective insulating coating after connection to the respective heat sink. One of the metallic heat sinks includes a finned area which is subjected to cooling air when the bridge rectifier is mounted to the generator. The heat sink with the plurality of fins includes twelve air passages. This type of bridge rectifier is shown in U.S. Pat. No. 4,606,000 to Steele et al., incorporated herein by reference.
However, this type of approach involves separate electronic packages for alternators of various sizes, e.g., six diodes vs. twelve diodes, and between light duty and heavy duty or off-road applications. For example a twelve diode rectifier cannot fit on a 114 mm diameter alternator designed for six diodes without the rectifier extending beyond the body of the machine. In addition, a six diode configuration does not have the flexibility to adapt to the thermal cooling requirements of a twelve diode rectifier or have brush lengths to match a given application.
Accordingly, there is a desire for an electronic package that can be used on small to large alternators (e.g., between about 114 mm to about 150+ mm stator diameter), six and twelve diode applications, and light duty and heavy duty applications. Moreover, there is a desire for an electronic package having a common footprint with a common layout and fastener locations.
On a typical alternator, a rotor shaft is supported in a drive end frame assembly by a front bearing and in a slip ring end (SRE) frame assembly by a rear bearing. The rear bearing includes an outer race fabricated of steel which is assembled with the slip ring end frame assembly fabricated of aluminum. Since steel and aluminum have different coefficients of thermal expansion (CTE), the fit between the outside diameter (OD) of the steel bearing and the inside diameter (ID) of the mating aluminum SRE frame housing the steel bearing changes with temperature. The size of the bearing OD and range of temperatures are such that having a direct press-fit between the bearing and SRE frame assembly will result in either having a radial press that is too high, and hence stress at this interface, at one temperature extreme, or a completely loose fit at the other extreme. Another requirement of this interface is that it must allow the bearing outer race to move axially allowing relative axial movement of the steel rotor shaft relative to the aluminum housing.
Currently, there are two general approaches to retain the rear bearing with the SRE frame assembly. One approach includes using a plastic material between the OD of the outer race of the bearing and the ID of the SRE frame assembly. Plastics have a greater coefficient of thermal expansion than aluminum and can effectively compensate for the radial mismatch between the aluminum and steel over the operating temperatures. Plastic, however, presents its own problems with respect to applied loads, elevated temperature, and time. Plastic creeps causing problems with radial alignment between the rear bearing and SRE frame assembly. Plastic also presents a thermal barrier to the outer race of the bearing preventing effective thermal conductance to the aluminum SRE frame assembly.
The other approach includes using a steel cup intermediate the bearing and the SRE frame assembly. However, because of the mismatch in thermal expansion between the steel cup and aluminum SRE frame assembly, the steel retaining cup typically includes multiple legs extending radially outward. These legs are then attached to the SRE frame assembly through threaded fasteners or similar features. Although effective from a product standpoint, such an arrangement is costly.
There is a need to improve the performance characteristics of prior art bridge rectifiers. In addition, there is a need for a SRE frame assembly interface with the rear bearing which increases the dissipation of heat from the rear bearing and more efficiently cool the rear bearing while limiting road splash at this interface. Furthermore, a more robust package assembly capable of assembly with six or twelve diodes while providing effective thermal dissipation of the rear bearing that is also cost effective is accordingly desired.